This contribution presents a dual-frequency molecular clock with trapped HD+ ions based on the two-photon rotational transition (v,L)=(0,1)->(0,3) at 3.268 THz and the two-photon rovibrational transition (v,L)=(0,3)->(9,3) at 207.634 THz which are detected by photodissociation of the (v,L)=(9,3) state. The two-photon transition rates between hyperfine components of HD+ energy levels and the lightshifts are calculated with the two-photon operator formalism. Temporal dependences of the populations of trapped HD+ ions are described by a set of coupled rate equations. The two-photon transitions may be detected efficiently with resolutions at the 10-13 level. The comparison, with an accuracy estimated at the 10-12 level, between experimental frequencies of two-photon rotational and rovibrational transitions of selected HD+ and H2+ ions and the values derived from quantum electrodynamics calculations may be exploited to determine the Rydberg constant, the proton-to-electron and deuteron-to-proton mass ratios, the proton and the deuteron radii independently on previous results. Depending on possible issues of the proton radius puzzle, measurements of hydrogen molecular ion two-photon transitions may improve the determination of the proton-to-electron and deuteron-to-proton mass ratios beyond the 10-11 level.

In this article, we illustrate a series of experiments performed in our group in the field of atom interferometry for precision gravity measurements. We show that instruments measuring and testing gravity can be built both with rubidium and with strontium atoms, while keeping the sources of systematic error under control. The application of these devices in the test of the Weak Equivalence Principle with quantum objects, in the measurement of the Newtonian gravitational constant G and in the development of a new type atom interferometer for the detection of gravitational waves is discussed.

This paper reports recent remarkable achievements of cold-atom technologies and related operational devices in the area of Quantum Sensing and Metrology.
We will present in detail the status of the Absolute Quantum Gravimeter (AQG) that has left the laboratory for geophysical studies. The AQG is an industry-grade commercial gravity sensor which today meets the objective to provide a gravimeter based on atom interferometry with laser-cooled atoms as a mobile turn-key device. We report on an operational stability of continuous absolute measurements of g at the 10-10 level in various types of environment during month-long continuous acquisition periods. The first unit of the AQG has traveled more than 7000 km, so we will comment on the last measurement campaigns and comparisons performed by the AQG. These have in particular validated the repeatability of the measurements at the 10-9 level, the ease of use and the robustness of such technology.
This paper will also be the occasion to describe in more details the maturity of several key technologies such as intelligent integrated laser systems that can help Quantum Technologies with cold-atoms taking-off for a wider range of applications in Quantum Computing, Quantum Simulation and Quantum Communication.

As recently demonstrated [T. Bagci, et al., Nature 507, 81 (2013)], an opto-electro-mechanical system formed by a nanomembrane, capacitively coupled to an LC resonator and to an optical interferometer, may be employed for the high{sensitive optical readout of rf signals. Here we show through a proof of principle device how the bandwidth of such kind of transducer can be increased by controlling the interference between the electromechanical interaction pathways of a two{mode mechanical system. The transducer reaches a sensitivity of 10 nV=Hz1/2 over a bandwidth of 5 kHz and a broader band sensitivity of 300 nV=Hz1/2 over a bandwidth of 15 kHz. We discuss strategies for improving the performance of the device, showing that, for the same given sensitivity, a mechanical multi-mode transducer can achieve a bandwidth significantly larger than that of a single-mode one.

We have shown that Rydberg atoms can be used for high-sensitivity, absolute sensing of radio frequency (RF) electric fields. We achieved a sensitivity of 3 μVcm-1Hz-1/2 for two read-out strategies. Results using a Mach-Zehnder interferometer2 and frequency modulated spectroscopy both achieve similar photon shot noise limited sensitivity. Fundamental limits to the sensitivity of the Rydberg atom-based RF electric field sensing have been addressed. Depending on the spectral resolution of the read-out, either the RF induced transmission line frequency splitting, the Autler-Townes regime, or a change in the on-resonant absorption, the amplitude regime, can be used to determine the RF electric field. Here, we present theoretical results of a 3-photon read-out scheme which enables the Autler-Townes regime of Rydberg atom-based RF electrometry to be extended to lower RF electric field strengths. We show that the residual Doppler shifts can, in principle, be reduced to ~11.8 kHz, on the order of the Rydberg atom natural lindewidths, using the approach.

Rydberg atoms in room temperature vapor cells are promising candidates for realizing new kinds of quantum devices and sensors. However, the alkali vapor, which is most commonly used, introduces new technological challenges. We demonstrate the applicability of anodic bonding as a sealing method for vapor cells, which preserves vacuum levels down to 10-7 mbar for several years, while being compatible with thin-film electronics on glass. We furthermore prove, that the implementation of such thin-film electronics inside a highly reactive atmosphere of alkali vapor is possible. We also propose a new kind of gas sensor based on Rydberg excitations as a competitive and promising application of our Rydberg detection scheme.

Temporal imaging is a technique enabling manipulation of temporal optical signals in a manner similar to manipulation of optical images in spatial domain. The quantum description of temporal imaging is relevant in the context of long range quantum communication. Indeed this technology relies on the efficiency of quantum repeaters for which the temporal mode matching between the quantum emitters, the communication network and the quantum memories is critical. In this work we address the problem of temporal imaging of a temporally broadband squeezed light generated by a traveling-wave optical parametric amplifier. We consider a single-lens temporal imaging system formed by two dispersive elements and a parametric temporal lens, based on a non- linear process such as sum-frequency generation or four-wave mixing. We derive a unitary transformation of the field operators performed by this kind of time lens and evaluate the squeezing spectrum at the output of the single-lens imaging system. When the efficiency factor of the temporal lens is smaller than unity, the vacuum fluctuations deteriorate squeezing at its output. For efficiency close to unity, when certain imaging conditions are satisfied, the squeezing spectrum at the output of the imaging system will be the same as that at the output of the OPA in terms of the scaled frequency ΩI = MΩ which corresponds to the scaled time tI = t/M . The magnification factor M gives the possibility of matching the coherence time of the broadband squeezed light to the response time of the photodetector.

The method of optical centroid measurement (OCM) has shown to exhibit spatial super-resolution with enhancements at the Heisenberg limit in plane wave interference experiments. In this work, the OCM method is for the first time used in an imaging setting where actual object features are observed. The OCM result is rederived in a near-field imaging formalism for a general imaging system and in full 2-D treatment. Analogies to coherent and incoherent imaging are shown Moreover, coherent OCM imaging is experimentally implemented for photon number N = 2, where an experimental setup is presented which allows to generate the desired entangled two-photon state containing the super-resolved image. This state is then imaged by an imaging system with a finite resolution defined by its point spread function (PSF). The centroid measurement of the two-photon states delivers then an image with a width of the PSF reduced by a factor 2 corresponding to the Heisenberg limit.
In the experiment, the object is illuminated by a continuous-wave pump laser centred at 405nm with an output power of 30mW. A 4-f lens system images the object to the state preparation output plane. In the far-field plane between the lenses, a 5mm long periodically poled KTiOPO4 non-linear crystal generates photon pairs at 810nm by type-0, frequency-degenerate and collinear spontaneous parametric down-conversion. This generated OCM state is then imaged by a single lens and detected in coincidence by a fully digital 2-D sensor array with single-photon sensitivity and per-pixel sub-nanosecond time resolution.
The OCM state is spectrally filtered at 810nm. Its imaging capability is compared to classical light sources including spatially coherent, monochromatic illumination at 405nm and 810 nm, as well as spatially incoherent light at 810 nm. The former is implemented using collimated lasers, the latter is a thermal light source spectrally filtered at 810 nm. The PSF of the different light sources are compared at low numerical aperture by imaging a focal point of 25μm Gaussian waist radius.

Plenoptic imaging (PI) is an optical technique to perform three-dimensional imaging in a single shot. It is enabled by the simultaneous measurement of both the location and the propagation direction of light in a given scene. Despite being very useful for extending the depth of field, such technique entails a strong trade- off between spatial and angular resolution. This makes the resolution and the maximum achievable depth of focus inversely proportional; hence, resolution cannot be diffraction-limited. We have recently proposed a new procedure, called Correlation Plenoptic Imaging (CPI), to overcome such fundamental limits by collecting plenoptic information through intensity correlation measurement. Using two correlated beams, from either a chaotic or an entangled photon source, we perform imaging in one arm and simultaneously obtain the angular information in the other arm. In this paper, we discuss the case in which the two correlated beams of light are generated by spontaneous parametric down-conversion. We review the principles of CPI with entangled photons and discuss its resolution and depth-of-field limits.

Efficient extraction of photons from quantum emitters is an important prerequisite for the use of such emitters in quantum optical applications as single photons sources or sensors. One way to achieve this is by coupling to a suited photonics structure, which guides away the emitter light. Here, we show the coupling of a single defect in hexagonal boron nitride (hBN) to a tapered optical fiber via a nanomanipulation technique [1]. Defects in hBN are capable of emitting single photons at room temperature while being photostable at the same time – two properties that make them ideal candidates for integration in single photon sources. The high control the manipulation technique provides avoids covering the whole nanofiber with emitters. We characterize the coupled system in terms of achievable count rates, saturation intensity, and spectral properties. Antibunching measurements are used to proof the single emitter nature of the defect. Our results pave the way for integration of single defects in hBN into photonic structure and their use as single photon sources in quantum optical applications such as quantum crypthography.
[1] A W Schell et al., ACS Photonics, 4, 761–767 (2017)

Efficient photon sources will enable many quantum technologies. Single dibenzoterrylene (DBT) molecules are promising photon sources, but often emit in an unknown direction making photon collection challenging. Dielectric structures redirect emission into single optical modes [1], but are relatively large due to the diffraction limit of light. Plasmonic devices, such as antennae, can concentrate the electromagnetic field at the site of an emitter on a surface in volumes below the diffraction limit and redirect emission into well-controlled directions, but often suffer from losses. Recently, planar dielectric antennae have shown promise for redirecting emission [2], however often they do not provide single mode operation or compatibility with integrated photonics.
Here we present a hybrid dielectric--metal approach in coupling a single molecule to an optical mode in an integrated planar device. We designed and fabricated a hybrid plasmonic waveguide (HPW) consisting of a dielectric slab with a nanoscale gap patterned in gold on the surface. Replacing the silicon layer used in our previous work [3] with titanium dioxide (TiO$_2$) allows operation at ~785 nm, the emission wavelength of DBT. Light propagating in the TiO$_2$ layer passes through the gap between the islands of gold. The width of the gap controls mode confinement: when the gap is <100 nm the propagating mode is mainly in the gap providing strong confinement; but when the gap is wider the mode decouples from the gold and propagates mainly in the TiO$_2$ with low loss. We deposited DBT-doped anthracene crystals on the surface using a supersaturated vapour growth technique [4]. Using confocal fluorescence microscopy we found a DBT molecule positioned near the gap. We then measured the saturation intensity of the molecule to be $I_{sat} = 325(27)$ kW/cm$^{2}$. Illuminating the molecule with a pulsed laser we measured the lifetime of the molecule to be 2.74(2) ns. Under CW excitation we measured the second-order correlation function $g^{(2)}(tau)$ of the light emitted directly into the microscope. This shows clear anti-bunching with $g^{(2)}(0)=0.25(6)$ proving this to be a single molecule. By detecting photons simultaneously from the microscope and from the grating coupler we measured $g^{(2)}(0)=0.24(6)$, demonstrating that this single molecule was emitting into the waveguide mode. By measuring the optical losses in our setup we calculated the coupling efficiency from the molecule to the HPW to be ~22%. This method provides a route to building waveguide sources of photons in planar integrated quantum photonic circuits for applications in quantum technology.
[1] S. Faez et al., Phys. Rev. Lett. 113, 213601 (2014).
[2] X. L. Chu et al., Optica 5, 203-208 (2014).
[3] M. A. Nielsen et al., Nano. Lett. 16, 1410-1414 (2016).
[4] C. Polisenni et al., Opt. Express 24, 5615-5627 (2016).

Photonic quantum devices based on atomic vapors at room temperature combine the advantages of atomic vapors being intrinsically reproducable as well as semiconductor-based concepts being scalable and integrable. One key device in the field of quantum information are on-demand single-photon sources. Comparable to similar realizations using cold atoms [1], it has been supposed to realize room-temperature single-photon sources by combining the two effects of four-wave mixing and Rydberg blockade [2]. Driving the four-wave mixing cycles in a pulsed manner, single photons are generated on demand.
The essential conditions of coherence [3,4] and sufficient Rydberg-Rydberg interaction strengths [5] have already been demonstrated. Up to now, the third condition has been missing: That is performing the excitation cycles in a low-dimensional geometry in order to obtain an excitation blockade of the whole volume by only one Rydberg excitation.
We realize this spatial confinement in transversal direction by focusing one of the excitation beams by means of a high-NA lens, and in longitudinal direction by using vapor cells of about one micrometer inner thickness (“µ-cells”).
In order to deal with reasonable numbers of atoms in such a small volume, we exploit the fact that in thermal equilibrium there are large numbers of adsorbed atoms on the glass surface. We can easily photo-desorb them shortly before the four-wave mixing cycles and increase by this the optical thickness in the excitation volume by one order of magnitude.
By means of these techniques we are now able to generate single photons at 780nm. With a Rydberg blockade radius of 1.0µm and a cell thickness of 1.2µm, we observe pure anti-bunching in the light statistics of the coherent emission field, deviating with a significance of three standard deviations from Poisson-type statistics.
We obtain generation efficiencies of currently up to 5% per four-wave mixing cycle, depending on experimental parameters.
We systematically investigate the disappearance of the anti-bunching by increasing the cell thickness. The results are also compared to measurements at smaller blockade radii.
This technique can be further improved by investigating different Rydberg states or different multi-level schemes which allow exploiting the latest developments in laser research.
[1] Y. O. Dudin et al., Science 336, 6083 (2012)
[2] M. M. Müller et al., PRA 87, 053412 (2013)
[3] B. Huber et al., PRL 107, 243001 (2011)
[4] B. Huber et al., PRA 90, 053806 (2014)
[5] T. Baluktsian et al., PRL 110, 123001 (2013)

In recent years there has been rapid progress into realising a working universal quantum computer, in particular with the development of chip-based radio frequency ion traps. The next significant leap will come with successfully integrating optical cavities into these ion traps to allow for interaction between remote ions via photons as required for more efficient and scalable quantum networking schemes. Fibre-tip cavities are especially interesting for such applications as they enable highly efficient coupling of photons from the cavity into optical fibres for onward transmission.
Here we analyse theoretically and numerically the effects of parallel off-axial misalignment in millimetre scale optical Fabry-Perot cavities. While near-concentric cavity configurations produce the smallest mode waist and thus strongest coupling to a trapped ion, their mode is extremely sensitive to misalignment. Shorter cavities exhibit more robust modes, but at the cost of larger mode waists. For example, for typical experimental parameters (mirror radius of curvature 0.7 mm, mirror diameter 0.140 mm, operation wavelength 850 nm) we find that the cavity lifetime is reduced by a factor 1/e for a misalignment of 0.95 nm for a beam waist of 2.91 um (cavity length of 1.397 mm), which increases to 11.0 nm for a waist of 4.33 um (length of 1.386 mm), and 3.12 um for a waist of 7.38 um (length of 1.273 mm). In the parameter regimes of interest, we derive a simple relation between cavity length, mirror radius, and misalignment sensitivity. Finally, we also consider the effect of mode matching of the misaligned cavity mode with the optical mode of the fibre for efficient cavity to fibre coupling.
In conclusion, our model allows us to optimise photon-ion coupling in fibre-tip resonators for quantum information processing in the presence of finite fabrication and alignment tolerances.

In quantum mechanics, the eigenvalues and their corresponding probabilities specify the expectation value of a physical observable, which is known to be a statistical property related to large ensembles of particles. In contrast to this paradigm, we demonstrate a unique method allowing to extract the expectation value of a single particle, namely, the polarisation of a single protected photon, with a single experiment. This is the first realisation of quantum protective measurements.

The model magnet LiY1-xHoxF4 has been shown to exhibit a variety of quantum many-body phenomena, such as quantum phase transitions, quantum annealing, long lived coherent oscillations and long-range entanglement, making LiY1-xHoxF4 a promising candidate for the implementation of solid state qubits. The magnetic moment of the Holmium atoms stems from the well screened f-shell electrons and the dynamics is largely dominated by dipolar interaction which can be tuned by doping concentration x. The energy levels of the rare-earth magnetic ion develop as follows: The degeneracy of the free-atom electron states arranged by the native strong spin-orbit interaction is lifted by the tetragonal crystal lattice symmetry (point group S4) and subsequently further split by the hyperfine interaction with the nuclear spin I=7/2.
Earlier work optically probed the transition from the eightfold hyperfine-split ground state to the second excited state in a Fourier transform infrared (FTIR) spectrometer with a lab infrared source and 1.2 m optical path difference (OPD), hence with limited signal to noise ratio and resolution. We present data using high brilliance synchrotron radiation light in the far infrared regime from the Swiss Light Source (SLS) at Paul Scherrer Institut in Switzerland taken with a high resolution FTIR spectrometer featuring 11 m OPD allowing us to probe the ground state to second excited state transition hyperfine lines with unprecedented precision of 0.00077 cm-1 which corresponds to 23 MHz. This precision allows us to extract the full width half maximum (FWHM) of the hyperfine linewidths as function of temperature and three different concentrations (x=0.3%, 0.25%, 0.1%). We observe Arrhenius behavior of the linewidths as a function of temperature and decreasing linewidths for decreasing concentrations. For the lowest doping x = 0.1% and T=6 K we find an average FWHM of 0.006 cm-1, which corresponds to 180 MHz and a lower bound lifetime of 0.46 ns.
As a next step, we push towards a more detailed examination of the absorption line shapes and intensities, and measurements of lower Holmium doping concentrations as well as other compounds with sharp absorption lines in the infrared regime.

Very recently, the interest for quantum technologies by the scientific community and industry has strongly increased. Different types of implementations have been proposed as a practical implementation for a quantum bit as trapped atoms, superconducting qubits and single photons. In particular, we are interested in using single photons and single spins in solid state host matrices such as diamond (nanodiamonds or membranes). Integration of nanosources of light is currently a major bottleneck preventing the realisation of all-photonic chips for quantum technologies and nanophotonics applications. Nanophotonics and integrated optics are vast growing fields with huge market potentials in particular for quantum technologies. Ideally, one needs optical circuitry, on-chip photodetection and on-chip generation of quantum states of light (single photons, entangled photons…). We want to present our recent work on using integrated optics that can offer an easier and robust way to create fixed and compact quantum circuits that can be on chip and scalable. In this context, the coupling between waveguides and single photon emitters is critical. The goal of our research is to efficiently couple single photon emitters with a new platform made of optical glass waveguides. To achieve this goal, several paths are undertaken such as the use of dielectric and plasmonic structuration in order to increase the light interaction with the waveguide or to develop fabrication techniques to insert the emitters directly inside the guide (for nanodiamonds). We will show what is our current state of the art for placing single emitters at the right place on our optical waveguides made of ion-exchange in glass and in particular what can be done to improve our first promising results in order to get near unity coupling between the optical bus and single photon emitters. We will show first results with semiconductor nanocrystals (NCs) but also using nitrogen-vacancy defects in diamond either under nanodiamond form or in thin membranes.

Controlled placement and excitation of quantum emitters in plasmonic nanostructures is highly desired for efficient plasmon-emitter interfaces on a chip. We present a hybrid approach for direct energy exchange between propagating surface plasmons and quantum emitters using low-loss dielectric-loaded surface plasmon polariton waveguides (DLSPPWs). Alignment of the waveguides axes is pre-determined to maximize emitter decay into fundamental DLSPPW mode. Lithographic fabrication is performed on negative high-resolution electron-beam resist to incorporate nanodiamonds containing single nitrogen-vacancy (NV) centers. Efficient grating couplers at the two ends of the waveguide facilitate input coupling of excitation light and output coupling of the NV emission. The hybrid platform provided a controlled activation of NV emitter micrometers away from the host nanodiamond and can be further expanded to include several quantum emitters, revealing the potential of our approach for realization of functional on-chip quantum plasmonics.

We investigate alternative focusing geometries to couple near-resonant light onto a single neutral atom. In particular, we show significant light-atom interaction using the ‘4Pi-microscopy’ configuration. Performing a transmission experiment, we find a resonant extinction of 36.6(3)%. Furthermore, photon anti-bunching in the second-order correlation function of the transmitted light demonstrates nonlinear light-atom interaction at the level of single photons. Our results indicate that free-space focusing provides an alternative route to realise nonlinear optics with single photons.

We present a new experiment, in which we measure quantum correlations between single photons that are mediated by the exchange of a single phonon. We create and annihilate a single optical phonon in bulk diamond using two ultrashort laser pulses at two different wavelengths, generated by a Ti:Sapph oscillator and a frequency-doubled optical parametric oscillator (APE Berlin). During Stokes Raman scattering, the first pulse creates a photon-phonon pair, while the second pulse convert the same phonon into an anti-Stokes photon. Using spectral filtering and photon counting, we measure the cross-correlations between the Stokes and anti-Stokes photons with a few hundred femtoseconds time resolution. As expected, the non-classical correlation (g(2) much larger than the classical bound) decays within a few picosecond, following to the dynamics of the phonon mode. Our results demonstrate a new source of broadly tunable quantum correlated photons, and can be extended to provide a new way of measuring non-classical dynamics in nanoscale systems — down to individual nanostructures.

We present recent findings towards developing brighter entangled photon sources in critically phase matched (CPM) nonlinear crystals. Specifically, we use type-I collinear phase matching at non-degenerate wavelengths in parallel β-Barium Borate (BBO) crystals to generate pairs of polarization entangled photons for free-space quantum key distribution (QKD). We first review the entangled source configuration and then discuss ways to further improve the source brightness by means of tailoring the pump and collection modes. We present preliminary results that may lead to brighter entangled photon sources that are intrinsically robust to changes in their environment.

Quantum key distribution (QKD) allows two users to communicate with theoretically provable
secrecy [1]. This is vitally important to secure the confidential data of governments, businesses
and individuals. As the technology is adopted by a wider audience, a quantum network will
become necessary for multi-party communication, as in the classical communication networks in
use today. Unfortunately, a number of phase-encoded QKD protocols based on weak coherent
pulses have been developed. Whilst the first protocol, proposed by Bennett and Brassard
in 1984 (BB84), is still commonly used, other protocols such as differential phase shift [2] or
coherent one way QKD [3] are also adopted. Each protocol has its benefits; however all would
require a different transmitter and receiver, increasing the complexity and cost of quantum
networks.
In this work we demonstrate a multi-protocol transmitter [4-6] that also has the benefits of
small footprint, low power consumption and low complexity. We use this transmitter to give the
first experimental demonstration of an improved version of the BB84 protocol, known as the
differential quadrature phase shift protocol. We have achieved megabit per second secure key
rates at short distances, and have shown secure key rates that are, on average, 2.71 times higher
than the standard BB84 protocol. This enhanced performance over such a commonly adopted
protocol, at no expense to experimental complexity, could lead to a widespread migration to
the new protocol.
The security of the BB84 protocol relies on each signal and reference pulse pair being globally
phase randomised with respect to all other pulse pairs. In the DQPS protocol, blocks with a
length L ≥ 2 are used and each block has a globally random phase with respect to all other blocks.
Implementing this protocol would ordinarily require a high-speed random number generator and
a phase modulator. As well as increasing device complexity, it would also require an unrealistic
continuous source of electrical modulation signals for complete security. The transmitter we
use injects light from a master laser diode into a 2 GHz gain-switched slave laser diode. The
principal of optical injection locking means that the slave laser inherits the phase of the master
laser. We apply modulations to the master laser current within a block to control the phase
of the slave laser output pulses, and then drive the master laser below threshold for a short
period of time when phase randomisation is required. This ensures the lasing comes from below
threshold, thus the phase adopted by the slave laser pulse is completely random. We perform
an autocorrelation measurement on the blocks to show their randomness.
[1] N. Gisin et al. Rev. Mod. Phys. 74, 145 (2002).
[2] K. Inoue et al. Phys. Rev. Lett. 89, 037902 (2002).
[3] D. Stucki et al. Appl. Phys. Lett. 87 194108 (2005).
[4] Z. Yuan et al. Phys. Rev. X. 6, 031044 (2016).
[5] G. L. Roberts et al. Laser Phot. Rev. 11, 1700067 (2017).
[6] G. L. Roberts et al. arXiv:1709.04214 [quant-ph] (2017).

We report an original all-optical synchronization scheme suitable for a quantum relay based experiments at telecom wavelengths. After discussing the entangled photon sources’ performances, we validate our scheme by performing a two-photon interference at the relay station.

As light propagates through a transmission media, such as an optical fiber, it experiences a length-dependent loss which can reduce the communication efficiency as the transmission distance increases. In conventional telecommunications, optical signals can be transmitted over inter-continental distances, due to deterministic all-optical amplifiers. However, quantum communications are still limited to transmission distances of typically a few 100’s km since deterministic amplifiers cannot be used to amplify quantum signals. The use of deterministic amplification on a quantum signal will introduce noise that will mask the original quantum properties of the signal, introducing uncertainty or errors to any measurement. Nondeterministic methods for amplifying quantum signals via post-selection can be used instead, providing a solution to create a low noise quantum amplifier. Several methods for nondeterministic amplification have already been experimentally demonstrated. However, these devices rely on “quantum resources” which makes implementation challenging. Here we present an overview of experimental demonstrations for amplifying coherent states using a method called state comparison amplification. This is a nondeterministic protocol that performs amplification of known sets of phase-encoded coherent states using two modular stages. The outcome of each stage is recorded using single-photon detectors and time-stamped electronics to enable post-selection. State comparison amplification is a relatively simple technique, only requiring “off-the-shelf” components. The presentation will show several demonstrations of state comparison amplification including an amplifier which has high gain, fidelity, and success rate with the added advantage of being robust to channel noise and easily reconfigurable. Finally, we will discuss the effect of introducing a feedforward mechanism allowing for unsuccessful state amplifications.

Optical Wireless Communication (OWC) systems rapidly increase their importance in very long-distance deep space communication scenarios. However, the high performance requirements of deep space OWC systems demand preliminary experiments which are unmanageable in real conditions. Regarding this issue, an innovative approach for testing deep space optical communication links in controlled laboratory environment is developed. The proposed testbed is based on fibre optics technology and combines various modules which represent a real deep space OWC link. Similar to already demonstrated deep space missions, the implemented optical receiver is Superconducting Nanowire Single- Photon Detector (SNSPD) characterized with single photon sensitivity and high detection efficiency. Consequently, in this paper an authentic deep space Poisson channel is emulated and examined. The given theoretical description of the Poisson process is supported by real SNSPD measurements in terms of high efficient single photon detection. The provided measured graphs clearly show the operation of SNSPD. In addition, a variable optical attenuator (VOA) is applied as a main device emulating the tropospheric part of a deep space optical Poisson channel characterized predominantly by Mie scattering (fog and clouds) and turbulence effects. This OWC channel emulator also contains self-developed software and attenuator control unit based on external Digital Analog Converter (DAC). Moreover, the response time parameter of channel emulator is examined in detail. Two different times in terms of reaching the lowest and the highest allowed attenuation are measured and shown. Finally, the developed channel emulator is tested and evaluated under real attenuation data. The experimental results show that the proposed method can evaluate various deep space optical scenarios.

There is considerable interest in finding conditions under which the quantum key distribution (QKD) propagation distances over fiber and secure key rate (SKR) are maximized for a given acceptable quantum bit error rate. One way to increase the secure key rate is to increase quantum bit rate, i.e. use shorter pulses. Short pulses propagating in a fiber are subject to temporal broadening caused by chromatic dispersion (CD) which leads to inter-symbol-interference and quantum bit-error rate increase. Current commercial QKD systems employ 1 Gb/s quantum bit rate sources, and the transition to 10 Gb/s system is being researched. While not very important in the 1 Gb/s, the effect of CD cannot be neglected in 10 Gb/s or higher quantum bit rate systems.

The capability to achieve high count rates has become an imperative in the most areas where near-infrared single-photon counters are required to detect photons up to 1.7 μm. Hence, afterpulsing mitigation is a dominant theme in recent works concerning systems based on InGaAs/InP SPADs. Given the challenges inherent in reducing the density of defects that give rise to the carrier trapping events causing afterpulsing, the only viable approach is to reduce the potential number of carriers that can be trapped by limiting the charge flow per avalanche event. In this paper we present a sine-wave gating system based on the balanced detector configuration. The gate frequency is programmable in a wide range (1.0 – 1.6 GHz) for allowing synchronization with an external laser system and for exploring the best trade-off between afterpulsing and photon detection efficiency. The long-term stability can be achieved with a stable cancelation of the gate feedthrough. In this work this is guaranteed by a feedback loop that continuously monitors the residual output power at the gate frequency and adjusts the amplitude and phase of the two sinusoids fed to the SPAD-dummy couple.

Quantum key distribution (QKD)1 is a quantum technology already present in the market. This technology will become an essential point to secure our communication systems and infrastructure when today’s public key cryptography will be broken by either a mathematical algorithm or by, eventually, the development of quantum computers. One of the main task of quantum metrology and standardization in the next future is ensuring that QKD apparatus works as expected, and appropriate countermeasures against quantum hacking are taken. In this paper, we discuss the security of one of the QKD most critical (and quantum-hackered) components, i.e., single photon detectors based on fiber-pigtailed InGaAs SPADs. We analyze their secondary photon emission (backflash light) that can be exploited by an eavesdropper (Eve) to gain information without introducing errors in the key. We observed a significant light leakage from the detection event of fiber-pigtailed InGaAs SPADs. This may represent a significant security threat in all QKD apparatus. We provide a method to quantify the amount of potential information leakage, and we propose a solution to fix this potential security bug in practical QKD apparatus.

Technological advances in quantum computers and number theory have the potential to compromise the security of existing cryptographic protocols. Quantum key distribution (QKD) offers the possibility of information theoretic security and is theoretically unbreakable. Therefore it is the natural candidate to face the above digital threat.
However, in implementing QKD, it is important to check that the components employed do not deviate from their expected behaviour, to avoid opening the door to new security loopholes [1]. For this reason, it is necessary to characterise the real behaviour of the components, build reliable models and include them in the security analysis.
Here we introduce a set of techniques and measurements to ease this characterisation process. We discuss explicit examples applied to the source [2], the boundaries [3] and the detection unit [4] of a QKD apparatus. These methods pave the way to the future certification of QKD systems.
[1] K. Tamaki, M. Curty, and M. Lucamarini, “Decoy-state quantum key distribution with a leaky source,” New J. Phys 18, 65008 (2016).
[2] J. F. Dynes et al., “Testing the photon-number statistics of a quantum key distribution light source,” arXiv:1711.00440 (2017).
[3] M. Lucamarini et al., “Practical Security Bounds Against the Trojan-Horse Attack in Quantum Key Distribution,” Phys. Rev. X 5, 031030 (2015).
[4] A. Koehler-Sidki et al., “Setting best practice criteria for self-differencing avalanche photodiodes in quantum key distribution,” SPIE Proc. 10442, Quant. Inf. Sci. Tech. III, 104420L (2017).

Quantum key distribution (QKD) is one of the most commercially-advanced quantum optical technologies operating in the single-photon regime. The commercial success of this disruptive technology relies on customer trust. Network device manufacturers have to meet stringent standards in order to ensure the operational security of their devices. The National Physical Laboratory (NPL) and the University of Bristol (Bristol) are working to produce a suite of tests to determine the operating characteristics and implementation security of chip-scale quantum devices designed for security purposes. These tests will inform and provide assurance to potential customers of such devices. Results from initial measurements performed on the Bristol chip-scale transmitter and receiver are presented, with the aim of informing the development of the system.

InGaAs/InP single-photon detectors are today the most frequently used detectors for fibre-based Quantum Key Distribution (QKD) [1,2]. The performance of the QKD-systems greatly depends on the optical properties of its detector, such as quantum efficiency, dead time, dark counts, gating rate, etc., which need to be metrologically characterized to fully grantee the QKD network security. Therefore, several European National Metrological Institutes (NMIs) are putting today great efforts in developing novel measurement methods and calibration facilities, which allow to perform the traceable characterisation of single-photon detectors by using reference standards [3, 4]. From the metrological, and specifically radiometric, point of view, the detection efficiency of the single-photon detector is the main parameter that needs to be traceable to the primary standard for optical power (i.e. the cryogenic radiometer), which is maintained by most of the NMIs.
Recently, four European NMIs have carried out a pilot comparison on the detection efficiency of a free-running InGaAs/InP single-photon detector, which main purpose has been to provide a snapshot of the measurement capabilities of the four European NMIs in the field of photon counting detection. The comparison was carried out in the framework of the EMPIR project 14IND05 “Optical metrology for quantum-enhanced secure telecommunication (MIQC2)” at the telecom wavelength of 1550 nm by using different reference standards with independently traceability chains. The results of this comparison, including the different experimental setups, the measurement methods, the traceability chain and the uncertainty evaluation of each participant, will be presented in this conference.
References:
[1] Akihisa Tomita, et al., “High speed quantum key distribution system”, Optical Fiber Technology, 16, Issue 1, 55-62 (2010).
[2] Damien Stucki, et al., “Photon counting for quantum key distribution with peltier cooled InGaAs/InP APDs”, Journal of Modern Optics, 48, Issue 13, 1967-1981 (2001).
[3] M. López, et al, “Detection efﬁciency calibration of single-photon silicon avalanche photodiodes traceable using double attenuator technique, Journal of Modern Optics 62, S21 – S27, 2015.
[4] G. Porrovecchio, et al., “Comparison at the sub-100 fW optical power level of calibrating a single-photon detector using a high-sensitive, low-noise silicon photodiode and the double attenuator technique”, Metrologia 53, 1115-1122 (2016).

Quantum mechanics imposes stringent constraints on the amplification of a quantum signal. Deterministic amplification of an unknown quantum state always implies the addition of a minimal amount of noise. Linear and noiseless amplification is allowed in principle provided that it only works probabilistically. Here we present a probabilistic amplifier that combines two quantum state comparison amplifiers (SCAMP) together with a feed-forward state correction strategy. Our system outperforms the unambiguous state discrimination (USD) measure-and-resend based amplifier in terms of the success probability-fidelity product and requires a more complex experimental setting.

The time-bin quantum state is known to be highly robust against decoherence effects in both fiber-optic and atmospheric channels, a unique feature that renders the time degree of freedom (DOF) more appropriate for quantum communication in these channels. In this paper, we present a scheme to deliver with high fidelity an arbitrary, unknown quantum state in polarization or spatial DOFs over a stochastic channel without need for compensation. The sender swaps the polarization or spatial quantum state for a time-bin state of the photon before signaling it over the random channel, and the receiver swaps the state back. Because the signaled photon is assumed to be in a single spatial or polarization mode, no modal-dependent channel effects perturb the time-bin state. We find that by migration to the time bin, the fidelity of the transferred state is boosted by a margin dependent on the time-bin period and the standard deviations of the statistical parameters of the channel.

Laser Induced Breakdown Spectroscopy (LIBS) is an analytical technique, used to classify and potentially quantify elements in complex hosts (or matrices). Vacuum Ultraviolet Laser Induced breakdown Spectroscopy (VUV LIBS) can offer potential improvements in detection of light elements in bulk metals over traditional LIBS in the visible region. This is due to presence of an abundance of resonance transitions at shorter wavelengths. This extends the ability to discriminate between the emission from different elements, particularly light elements such as carbon, sulfur, lithium, beryllium etc. Additionally, the precision of LIBS is limited by the continuum emission at the early stage of the plasma lifetime. The performance of LIBS can be improved by using a time resolved detection system [5], reducing the contribution from the continuum. In this study, the detection of the carbon content in steel samples is performed by time- integrated and time-resolved VUV LIBS. The experimental setup consists of a dual pulse system with Nd:YAG laser (1064 nm, up to 450 mJ, pulse duration 6 ns) used to irradiate the samples, a vacuum system to prevent absorption of the VUV radiations and a VUV spectrometer to collect and measure the emission spectra. Samples of four different concentrations of carbon in steel are used for the study. The resultant time integrated LIBS limit of detection and signal to background ratio is compared with time resolved VUV measurements.

We present an on-chip nonlinear optics based correlated and higher dimensional state photon source using silicon hybrid materials. The four-wave mixing process occurring in a ring resonator is used to generate a frequency comb of signal and idler wavelengths corresponding to different resonant wavelengths around the pump resonance. The frequency comb based four-wave mixing process is used to generate higher-dimensional entangled photon pairs. The individual comb lines, into which the correlated photon pairs could be generated leads to higher dimensional entanglement. The ability to generate higher dimensional photon states is advantageous to pack more information for high data rate quantum communication and information processing applications.

Quantum Key Distribution, a fundamental component of quantum secure communication that exploits quantum states and resources for communication protocols, can future-proof the security of digital communications, when if advanced quantum computing systems and mathematical advances render current algorithmic cryptography insecure. A QKD system relies on the integration of quantum physical devices, as quantum sources, quantum channels and quantum detectors, in order to generate a true random (unconditionally secure) cryptographic key between two remote parties connected through a quantum channel. The gap between QKD implemented with ideal and real devices can be exploited to attack real systems, unless appropriate countermeasures are implemented. Characterization of real devices and countermeasure is necessary to guarantee security. Free-space QKD systems can provide secure communication to remote parties of the globe, while QKD systems based on entanglement are intrinsically less vulnerable to attack. Metrology to characterize the optical components of these systems is required. Actually, the “Optical metrology for quantum-enhanced secure telecommunication” Project (MIQC2) is steering the metrological effort for Quantum Cryptography in the European region in order to accelerate the development and commercial uptake of Quantum Key Distribution (QKD) technologies. Aim of the project is the development of traceable measurement techniques, apparatus, and protocols that will underpin the characterisation and validation of the performance and quantum-safe security of such systems, essential steps towards standardization and certification of practical implementations of quantum-based technologies.

Quantum superpositions of coherent states are produced both in microwave and optical domains, and are considered realizations of the famous “Schroedinger cat” state. The recent progress shows an increase in the number of components and the number of modes involved. Our work gives a theoretical treatment of multicomponent two-mode Schroedinger cat states. We consider a class of single-mode states, which are superpositions of N coherent states lying on a circle in the phase space. In this class we consider an orthonormal basis created by rotationally-invariant circular states (RICS). A two-mode extension of this basis is created by splitting a single-mode RICS on a balanced beam-splitter. After performing a symmetric (Loewdin) orthogonalization of the sets of coherent states in both modes we obtain the Schmidt decomposition of the two-mode state, and therefore an analytic expression for its entanglement. We show that the states obtained by splitting a RICS are generalizations of Bell states of two qubits to the case of N -level systems encoded into superpositions of coherent states on the circle, and we propose for them the name of generalized quasi-Bell states. We show that an exact probabilistic teleportation of arbitrary superposition of coherent states on the circle is possible with such a state used as shared resource.

Quantum Key Distribution (QKD) systems usually exploit the polarization of light to encode bit values, thus limiting to 1 bit the amount of information carried by each photon and placing serious limits on the error rates tolerated. Here we propose the use of two mutually unbiased bases for high-dimensional QKD that exploit the transverse spatial structure and coherence properties of the light field, allowing for the transfer of more than 1 bit per photon. Our proposed method employs coherence modulation with an orthonormal basis of time delays (TD) and the corresponding mutually unbiased basis of wave trains (WT). We construct the mutually unbiased basis set WT using a linear combination of orbital angular momentum OAM modes. Through the use of a high-dimensional alphabet encoded in the TD and WT bases, we achieve a high channel capacity of bits per inspected photon. In addition to exhibiting increased channel capacity, multidimensional QKD systems based on spatiotemporal encoding may be more resilient against intercept-resend eavesdropping attacks. Numerical simulations are presented as tests of the proposed QKD system. Experiments remain to be conducted to verify the concept.

The optical properties of biexciton and exciton states in strongly oblate ellipsoidal quantum dot are investigated in the framework of variational method. The trial wave function for the biexciton is constructed on the base of one-particle wave functions. The dependencies of the photoluminescence spectra for the crossover states of biexciton and exciton are constructed as a function of the incident light energy. The peak position of the photoluminescence spectra for biexciton and exciton are revealed. The oscillator strength of biexciton and exciton are calculated. The biexciton’s and exciton’s radiative lifetimes in strongly oblate ellipsoidal quantum dot are estimated.

We present an alignment procedure which allows for precise gluing of a structure with an optically pumped quantum emitter to the end face of zirconia ferrule with a specially fabricated high numerical aperture single-mode fiber. The proposed method is an important step towards building a single-photon source based on an InGaAs quantum dot emitting in 1.3 μm range and located deterministically in a microlens fabricated by in-situ electron beam lithography and plasma etching to improve the photon extraction efficiency. Since single QDs are very dim at room temperature which hinders QD-fiber adjustment by maximizing the collected photoluminescence signal, the developed method uses light back-reflected from the top surface of the sample with microlens as a feedback signal. Using this approach, we were able to position the high-NA fiber over the center of the microlens with an accuracy of about 150 nm in a lateral direction and 50 nm in a vertical direction. The alignment accuracy was confirmed by following the room temperature emission from quantum wells embedded in a reference microlens. We also present initial low temperature tests of the coupling system mounted in a compact and portable Stirling cryocooler.